The present invention relates to a transit time or so-called time-of-flight mass spectrometer (TOFMS) for analyzing ions having different energies, particularly ions emitted under the influence of radiation, especially laser radiation, which mass spectrometer is of the type having a device forming a drift path as well as an ion detector located at the output or downstream end of the drift path for analyzing the ions.
Transit time mass spectrometry is used for analyzing various elements and isotopes with respect to their mass to charge ratio. The specimen to be analyzed is subjected to stimulation such as laser radiation, ion radiation, electron radiation, or any other forms of energy application which causes the specimen to emit ions. These ions are accelerated in an electric field and are directed toward a drift path whose downstream output end leads to an ion detector or analyzer. Different ions are accelerated to different velocites, depending or their mass to charge ratio, and, after having travelled through the drift path at a constant velocity, reach the ion detector at different times. A brief start pulse, the duration of which must be smaller than the time interval between two consecutive masses, is obtained either by pulsed excitation or by a brief deflection of the beam in an electric or magnetic field.
The electrostatic lens system of a mass spectrometer of the above type is conventionally constituted by a unitary lens. The use of such a lens, however, makes it necessary to provide a suction field for drawing the ions in the direction of the lens, this normally being brought about with the help of a grid which is located in front of the specimen and which has the acceleration potential applied it. Such an arrangement has the drawback that the grid greatly attenuates the transmission, i.e., the ion flow, the reason for this being that the solid portions forming the meshes of the grid have finite widths. Still another drawback of such an arrangement is that even small deformations of the grid will cause aberrations or image distortions.
A mass spectrometer of the above general type is described in The Review of Scientific Instruments, Volume 37, No. 7, 196, pages 938 ff, which deals with a mass spectrometer for detecting the ions emitted as the result of being subjected to laser radiation. However, the mass spectrometer there described does not suggest the provision of a lens arranged ahead of the drift path so that the ions cannot be analyzed on the basis of their energy levels.
It is, therefore, one of the primary objects of the present invention to provide a transit time mass spectrometer of the above-discussed general type but which avoids the mentioned drawbacks.
Other objects to be accomplished by the present invention will be set forth below.